1. Field of the Invention
This invention relates generally to a vehicle wheel slip control system, and specifically to a vehicle wheel slip control system effective during a vehicle turn.
2. Description of the Prior Art
General vehicle antiskid systems adjust brakes to prevent vehicle wheel lock up or unacceptable vehicle wheel slip during applications of the brakes. In these antiskid systems, a vehicle speed and a vehicle wheel rotational speed are compared to determine the vehicle wheel slip ratio. Specifically, brake control to prevent vehicle wheel lock up is performed when the vehicle wheel rotational speed is lower than the vehicle speed by a reference value corresponding to an unacceptable slip ratio. Generally, vehicle wheels are independently exposed to such brake controls.
During vehicle turns, rotational speeds of vehicle wheels at one side of the vehicle differ from rotational speeds of vehicle wheels at the other side of the vehicle. Such a difference in rotational speed between the vehicle wheels tends to cause unreliable operation of the antiskid system.
Japanese published unexamined patent application 49-109789 discloses an improved antiskid system provided with a sensor detecting a vehicle turn. In this improved antiskid system, during a vehicle turn, reference values for determining the vehicle wheel slip ratios are corrected in accordance with output of the vehicle turn sensor to maintain reliable antiskid operation. The improved antiskid system is complicated, since the vehicle turn sensor is added.
In cases where diameters of vehicle wheels are different due to causes such as a difference between air pressures within the tires and a difference in wear between the tires, rotational speeds of the vehicle wheels are liable to differ from each other even when the vehicle is required to travel straight. This difference in the vehicle wheel rotational speeds also causes unreliable antiskid control. Even the improved system of Japanese patent application 49-109789 can not prevent such unreliable antiskid control.